1. Field of the Invention
This disclosure relates a system and method of installing a topsides on an offshore structure. More specifically, the disclosure relates to a system and method for installing modules by lifting or by a floatover technique onto an offshore structure, such as a Spar.
2. Description of the Related Art
Many offshore structures require a topsides to provide support facilities to operate the offshore structures for their intended purpose. For example, a Spar platform is a type of floating oil platform typically used in very deep waters and is among the largest offshore platforms in use. A Spar platform includes a large cylinder or hull supporting a typical rig topsides. The cylinder however does not extend all the way to the seafloor, but instead is moored by a number of mooring lines. Typically, about 90% of the Spar hull is underwater.
Deck or topsides installation has always been a challenge for floating structures, particularly in deep draft floaters such as the Spar, which must be installed in relatively deep water. In the past, topsides for most of previous floating structures and particularly, Spar platforms, were installed by heavy lift derrick barges in single or multiple module lifts ranging from about 3000 MT (metric ton) to over 30000 MT. There are some disadvantages in a lifted installation, particularly for large topsides requiring multiple module lifts. In traditional efforts, the topsides requires multi-lifting, for example five to seven lifts, to install the whole topsides due to the lifting capacity of available heavy lifting vessels. Such multi-lifts increase the costs and time for installing the topsides to the offshore structure.
As the offshore industry has matured and deeper wells are drilled, the needs of the industry have changed. There is significant market pressure to provide larger and more sophisticated topsides to meet the current demands of customers. One logical solution is to build larger and heavier capacity derrick barges for handling such massive topsides, despite the expense.
Floatover methods offer an alternative to the heavy left derrick barges. In general, one or more barges are loaded with a topsides near land and floated to the offshore structure, where the barge(s) or the offshore structure is ballasted so that the topsides can be transferred from the barge to the offshore structure. In some applications, a single barge is used that can be floated between support structures, the barge is ballasted and lowered in the water to allow the topsides to be fastened to the support structures. Recently, catamaran float-over systems have been used to install a topsides onto a Spar hull and other offshore structures. A catamaran float-over method is a concept in which the topsides is loaded onto at least two separated float-over barges to form a catamaran system and transported with the float-over barges to the installation site for the Spar hull. At the installation site, the float-over barges are positioned on both sides of the Spar hull with the Spar hull below the topsides, the elevation is adjusted between the topsides and the Spar hull, as the topsides is installed to the Spar hull.
However, as customers demand larger offshore topsides, the ability to load the massive topsides onto the barges as the topsides is pushed onto the barges, and the ability of the barges to float such topsides to the installation site is becoming more difficult. One solution is to build larger and heavier capacity barges and cranes.
A related, but more insidious, challenge is the assembly and planning required for such massive sizes. For example, at present, there is about a three-year waiting period for availability of offshore heavy lifting barges and equipment. Thus, scheduling of a lift is done often before even the topsides design is complete and certainly before the topsides is built that usually includes design changes. If the topsides is delayed through unavailability of components or other causes, then the window of the scheduled lift date that was predicted possibly three years in advance can be missed. Missing the scheduled date can cause a significant impact on the final completion of the project, because a new suitable installation window can be hard to obtain. Further, the design of the topsides is in some aspects dependent on the type of lifting method used—heavy lift or floatover. By an early commitment of an installation method, the associated cost is very dependent on market predictions and availability for activities two to three years into the further. Thus, the lift scheduling and the interdependent topside design imposes a large risk for the cost and completion schedule.
Further, the topsides is generally assembled and commissioned onshore, before it is installed by the heavy lifting or floatover methods onto the offshore platform. Thus, the use of the topsides on the offshore platform is dependent upon the completion of the entire topsides, even if certain functions of the topsides could be used in an early stage. The onshore assembly and related steps increase efficiency and reduce overall costs compared to assembling each component on the offshore platform. However, the onshore assembly and other steps cause this inflexibility and delay of use.
There remains then a need for an improved system and method of loading a topsides onto an offshore platform that is more efficient and flexible than current methods, while allowing the topsides to enjoy the benefits of onshore assembly.